A significant change in intracellular acidity occurs in Balanus photoreceptors. It is the aim of the present project to: a) Measure the time course of this change in pHi. This will necessitate improvement in the time resolution of the methods and to monitor pHi. b) Determine the origin of the photo-induced proton generation. Selective block of different metabolic pathways would provide a method to isolate the source. The rate of Cai in photoreceptor function will be investigated with a Ca-selective microelectrode.